In known variators of the toroidal-race rolling-traction type, it is well known to mount each roller in a roller carriage and to connect that carriage to a hydraulic ram subject to a controlled hydraulic force which generates opposing tangential forces (through sheer of the oil films) at the interfaces between the roller and the opposing discs.
A two-regime CVT using this type of variator, is described in detail in patent specifications GB-A-2023753 and GB-A-1078791 for example. The variator in this case comprises two input discs or rotors both of which rotate with an input shaft driven by a prime mover. One of these two discs is fixed to the shaft while the other is splined to the shaft so that the disc has freedom for limited displacement in an axial direction. An output disc or rotor lies between the two input discs and two sets of rollers make rolling contact between part-toroidal races formed on the adjacent faces of the input discs and the output disc.
In addition to its function as an input member, the axially displaceable end disc also serves as a piston operative to exert an end load on the variator. This is required to generate the necessary contacting forces between the rollers and the discs to transmit drive. If the variator is subjected to excessive end load efficiency will be low and component life short whereas, if it is inadequate, unacceptable slipping can occur at the roller-disc interface which will result in a loss of traction, a clearly undesirable effect.
GB-A-1 600 974 discloses a variator of the toroidal-race rolling-traction type in which an attempt has been made to compensate for an excess in end loading when the variator components are operating at speed. In this known variator, the outer end of the axially displaceable input disc carries a cylindrical casing divided internally into two chambers and each of these chambers houses one of two annular pistons mounted in tandem on the variator shaft. Each piston separates the relevant chamber into two cavities, one on either side of the piston, and in operation of the variator, the cavity pairs on the rotor side of the pistons are fed by a high pressure fluid while the cavity pairs on the other side of the piston are fed by a low pressure to produce a net end load on the variator as above described. By pressurising both sides of the end-load pistons in this way, the significant radial pressure gradient built up by the fluid in the high pressure cavities when the variator components are rotating at speed will be largely compensated for by the almost identical pressure gradients built up in the fluid present in the low pressure cavities. This enables the effective end-load to be substantially that which would apply if the centrifugal effect were not present i.e. if the end disc and its casing were not rotating. Thus the end loading assembly is what may be referred to as "centrifugally compensated".
In the end load assembly disclosed in GB-A-1600974, the low pressure fluid for the end loader cavities is accessed from the rotor/roller lubrication circuit whilst the high pressure fluid is accessed from the same source as that used for the single-acting roller-control pistons. Consequently, unless the supply pressure is extremely low, it is still possible for the magnitude of the differential end-load to be below that required to maintain traction.
In a variator of the kind above described, the ratio between the tangential (traction) force TF and the normal (perpendicular) force NF at the roller-disc interface is known as the traction coefficient TO. Above a certain value, referred to as the limiting traction coefficient, the roller will slide and traction will be lost. This means that with the system of GB-A-1 600 974, full centrifugal pressure compensation can only be used if the low pressure supply is at very low pressure.